A. Field of the Invention
The present invention relates to an improved system and technique for radially positioning a transducer relative to a rotating disc and for maintaining the transducer in the desired radial position during the rotation of the disc.
B. Prior Art
In magnetic recording and reproducing systems, rotatable magnetic discs having a number of concentric data tracks located on them are often used. While the disc is rotating, it is essential that the magnetic transducer used to either record the data on the disc or to reproduce it from the disc remain in precise alignment with the proper data track on the disc.
A primary requirement is that the servo data take up a minimum of the disc recording surface, since that area will be unavailable for the storage of other information. Among other techniques, it has been known to intersperse servo tracks with the other information recorded on the disc. This other information is recorded in tracks radially located between the radial locations of the servo data tracks. A technique that has previously been used, and which is used with the present invention, is to provide the encoded servo data on such tracks in narrow radial sectors located on the disc surface. The remainder of the disc surface is then available for the storage of other information on data tracks. The interspersed or "embedded" servo data is encoded on the magnetic disc so that any particular servo or data track can be uniquely identified from the signals produced by a magnetic transducer sensing the servo data.
The recorded servo data must really perform two functions. It must assist the transducer positioning mechanism in accessing a selected data track and it must provide a means for enabling the transducer to follow the selected track with as little deviation as possible from the track during the rotation of the disc.
In the system described in U.S. Pat. Nos. 4,027,338 and 4,032,984, a transducer that is radially positioned over a selected track on a magnetic disc and maintained in that radial position through the use of servo data embedded within radial sectors on tracks, as mentioned above, is described. In that system, and in other similar systems, track identification information necessary for positioning the transducer over the correct track is provided by a plurality of adjacent cells in which a signal is recorded in either one of two positions. These signals actually represent reversals of magnetic flux on the disc when typical magnetic gap detectors are used. The presence of a reversal of magnetic flux or a magnetic transition at one location within the cell is taken to represent a binary digit zero while the occurence of the reversal or transition at the second location within the cell indicates the binary digit one. The preferred code in that system is the Gray code so that 2.sup.N tracks can be uniquely encoded where N is the number of cells provided for the track identification function in each servo track. The detection of this information provides coarse servo data that is used to position the transducer in approximate registration with a selected target track.
Additional information encoded on the servo track serves to enable the transducer to follow the proper track during the rotation of the disc. This fine positioning servo data comprises a magnetic transition located at either one of two positions, the positions alternating with adjacent tracks. It will be appreciated that the system described in these patents and other similar systems that have been used use separate servo data for the track accessing and track following functions.